Automobile vehicle bodies are mainly assembled by joining pressed steel sheets together using spot welding. In spot welding used to assemble vehicle bodies, there are demands both to secure a nugget diameter according to the sheet thickness, and to suppress the occurrence of sputter.
Sputter includes internal sputter (a phenomenon in which parent metal that has been melted by welding is scattered from the superimposed faces of the steel sheets) and surface sputter (a phenomenon in which parent metal that has been melted by welding is scattered from contact surfaces between the steel sheet and the electrodes). In both cases, the surface quality of the vehicle body is impaired by the sputter scattering and adhering to the automobile vehicle body. Sputter adhering to moving parts of a welding robot is also a cause of equipment malfunction. Needle shaped surface sputter remaining on the spot weld surface causes damage to automobile wiring harnesses and the like, and therefore needs to be ground off with a grinder. There is accordingly a demand to avoid internal sputter and surface sputter, as well as to secure a specific standard nugget diameter in order to secure the required weld joint strength.
In vehicle body assembly, sometimes the nugget diameter is less than the standard nugget diameter due to various disturbance factors such as electrode wear, current forking to an existing weld point, gaps between pressed components, and the like. It is accordingly often necessary, when welding on a production line so as not to cause sputter to occur, to make the appropriate current range 1.0 kA or greater, or 1.5 kA or greater in test sample level evaluation.
Recently, there is increasing use in automobile assembly of resistance spot welding machines that use an inverter direct current method in place of a single phase alternating current method. Inverter direct current methods enable the transformer to be made small, with the advantage that installation to a robot that only has a small carrying weight capacity is possible, and so inverter direct current methods are often used particularly on automated lines.
Inverter direct current methods have a high heat generation efficiency due to imparting a continuous current, without switching the current ON and OFF as in conventionally employed single phase alternating current methods. There are, accordingly, reports of forming nuggets of the standard nugget diameter or greater using a low current, and of wider appropriate current ranges for than when using a single phase alternating current, even in cases in which zinc plated thin soft steel sheet material is employed that does not readily form nuggets.
In spot welding, as illustrated in FIG. 1, often resistance spot welding is employed in automobiles using a single stage current pass in which electricity is only passed one time. In FIG. 1, I on the vertical axis is the weld current, and t on the horizontal axis is time (the same applies to FIG. 2 to FIG. 7). However, the current value at which internal sputter is generated by welding using a single stage current pass is low with high tensile steel sheets using an inverter direct current method, and there is significant narrowing of the appropriate current range.
Japanese Patent Application Laid-Open (JP-A) No. 2010-188408 (sometimes referred to below as “Document 1”) describes a method to suppress the generation of sputter in spot welding of high tensile steel sheets by, as illustrated in FIG. 2, employing a two stage current pass method in which the main current pass is performed after the conformity has been raised between contacting faces of steel sheets by using a preparatory current pass.
JP-A No. 2003-236674 (sometimes referred to below as “Document 2”) describes a method to suppress the generation of sputter in spot welding of high tensile steel sheets by, as illustrated in FIG. 3, employing a current pass method in which a preparatory current pass is employed to raise the conformity between contacting faces of steel sheets, current passing is then stopped, and then the main current pass is performed.
JP-A No. 2010-207909 (sometimes referred to below as “Document 3”) describes employing a current pass method in which, as illustrated in FIG. 4 and FIG. 5, a preparatory current pass is employed to raise the conformity between contacting faces of steel sheets, the current value is then lowered, and then the current value is raised again, and main current pass is performed at a constant current, or a pulse main current pass is performed. The suppression of sputtering from occurring thereby in spot welding of high tensile steel sheets is described therein.
JP-A No. 2006-181621 (sometimes referred to below as “Document 4”) describes a method to suppress sputtering from occurring in spot welding of high tensile steel sheets by spot welding as the current value is increased, while repeatedly raising and lowering the current, as illustrated in FIG. 6.
A document “ISO 18278-2 Resistance Welding and Weldability—Part 2 Alternative procedure for the assessment of sheet steels for spot welding” (sometimes referred to below as “Document 5”) describes a spot welding method in which, as illustrated in FIG. 7, for steel sheets of sheet thickness 1.5 mm or greater, six cycles (120 milliseconds) or more of current pass and two cycles (40 milliseconds) of rest are repeated three times or more.